Was the underside of the top handle absolutley fileld with shite? Is it shiney clean now? I've noticed a louder noise on refurbished machines as the airflow can whip around as it's supposed to, which makes a louder noise. The fact that you say this only happens with the cyclone on denotes that only the cyclone can be the cause.
Do you have the rubber seal present on the side of the cyclone where it meets the chassis of the machine?
Do you have the rubber seal present that sits right at the bottom, sealing itself up to the bin flap?
Do you have the rubber seal present that sits around the cyclone top when you push it into the machine? Has it rolled itself out of its groove? A smear of oil on the seal (or saliva if it's clean inough and you dont care) helps it stay seated as it slips down.
With the cyclone off, if you block the suction from the chassis with your hand, does the noise occur? You should hear the motor pitch change, then the release valve in the sponge filter housing will open allowing air to escape.